Generator with dual cycloconverter for 120/240 VAC operation

ABSTRACT

A generator system in accordance with the invention has two modes of operation, such as 120 VAC and 240/120 VAC modes of operation. The generator system has a permanent magnet generator with two independent sets of windings that each generate a three phase AC voltage. One three phase AC voltage is coupled to a first or master cycloconverter and the second three phase AC voltage is coupled to a second or slave cycloconverter. Live outputs of each cycloconverter are coupled to each other through a switch, such as a relay and neutral outputs of each cycloconverter are coupled to ground. A controller controls the cycloconverters to provide a first voltage, illustratively 120 VAC, across their respective outputs having the same amplitude. When in the 120 VAC mode, the switch across the live outputs of the first and second cycloconverters is closed, shorting the live outputs of the first and second cycloconverters together and the controller operates the first and second cycloconverters so their output voltages are in phase with each other. When in the 240/120 VAC mode, the switch across the live outputs of the first and second cycloconverters is open and the controller operates the first and second cycloconverters so that their output voltages are 180 degrees out of phase. The permanent magnet generator has rotor position sensors that are used by a brushless DC motor drive to drive the permanent magnet generator as a brushless DC motor to start the engine of the generator system and also to develop cosine wave information for use in controlling the cycloconverters.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/440,959, filed on Jan. 17, 2003.

FIELD OF THE INVENTION

[0002] The present invention relates to portable generators, and moreparticularly, a portable generator using cycloconverters that has a 120VAC mode of operation and a 240/120 VAC mode of operation.

BACKGROUND OF THE INVENTION

[0003] Present day portable generators typically make use of asynchronous alternator or cycloconverter for providing the desired poweroutput, which is typically 120 VAC or 240 VAC. Important considerationsfor any portable generator are:

[0004] Voltage regulation;

[0005] Dual voltage output capability;

[0006] Idle voltage and frequency;

[0007] Frequency tolerance;

[0008] Harmonic distortion:

[0009] Induction motor operation

[0010] Charger operation

[0011] Grounding configuration;

[0012] 4-blade (120-240 volt) twist-lock compatibility;

[0013] Response to load changes; and

[0014] Size and weight.

[0015] With regard to idle voltage and frequency, it is far easier toprovide 120 volts and 60 Hz at idle using electronic solutions (i.e.,inverter technology) than it is with synchronous alternators. However,sufficient voltage “head room” is still required. This higher voltagerequires more turns in the alternator coils resulting in an increasedcoil resistance and reduced system efficiency.

[0016] Harmonic distortion present in the output waveform of a portablegenerator is another important consideration that must be addressed.While waveform purity is of little importance to constant speeduniversal motor-powered portable power tools, it is an importantconsideration when running induction motors and chargers. Inductionmotors will run on distorted waveforms, but the harmonic content of theinput will be converted to heat, not torque. The extra heating from theharmonics must be quantified if a inverter topology which produces adistorted waveform is to be implemented. A sine wave pulse widthmodulated (PWM) inverter will produce excellent waveforms with only somehigh frequency noise, but they are likely to require full H-bridgeswhich, traditionally, have not been easily adaptable to the NorthAmerican grounding convention and the 4-blade twist-lock wiringconvention.

[0017] With regard to grounding configurations, in North America, thestandard grounding convention requires that one side (neutral) of each120 volt circuit is grounded. This means that 240 volt circuits havefloating grounds.

[0018] Still another important consideration is 4-blade (120-240 volt)twist lock compatibility. This convention requires four wires: ground,neutral, 120 volt line 1 and 120 volt line 2. Each 120 volt circuit isconnected between a 120 volt line and neutral. The 240 volt circuit isconnected between the 120 volt line 1 and the 120 volt line 2.

[0019] The ability of a generator to respond to load changes is stillanother important consideration. All inverter topologies will provide afaster response to load changes than a synchronous alternator, due tothe large field inductance used by a synchronous alternator.

[0020] Concerning size and weight, it would also be desirable to makeuse of inverter topology because virtually any inverter topology willprovide size and weight benefits over that of a synchronous alternator.However, trying to produce sine waves from a two half bridge circuit mayrequire large capacitors that would reduce the benefit of volumereduction provided by the inverter topology.

[0021] Cycloconverters have been used in generator systems to convertthe AC voltage generated by the generator to the desired AC outputvoltage. Electrical systems using cycloconverters typically have an ACvoltage source to the cycloconverters that is fairly stiff (low sourceimpedance). Consequently, the AC phasing information for commutation ofthe SCRs of the cycloconverters can be directly derived from the 3-phaseAC voltages provided to the cycloconverters. Suitable filtering isnecessary to remove the commutation notches introduced by SCRswitching/commutation. However, permanent magnet generators provide avery soft AC source in that they have significant series reactance. Thispresents two problems for control of the SCRs of the cycloconverter in agenerator systems using a permanent magnet generator. First, the ACvoltage waveforms are significantly disturbed by the switching of theSCRs of the cycloconverter and thus would require significant filtering.Second, the reactance of the permanent magnet generator introduces asignificant phase shift between the back-emf voltage waveforms of thepermanent magnet generator (which cannot be measured) and the ACvoltages at the outputs of the permanent magnet generator (terminalvoltages), especially as the generator system is loaded. This loaddependent phase shift can't be eliminated by a simple filter.

[0022] Generators having two isolated 120 VAC outputs that can beswitched between 120 VAC parallel connection mode (120 VAC mode) to a240 VAC series connection mode (240/120 VAC mode) would typically use amulti-pole switch, as shown in FIG. 10. With reference to FIG. 10,generator system 1000 is shown as having two isolated 120 VAC sources1002, 1004, which could be cycloconverters such as cycloconverters 42,44 described below. Generator system 1000 also has a 120 VAC output,shown illustratively as resistance 1006, a 240 VAC output, shownillustratively as resistance 1008, and a switch 1010 that switchesgenerator system 1000 between the 120 VAC parallel connected mode wheresources 1002 and 1004 are connected in parallel and the 240 VAC seriesconnected mode where sources 1002 and 1004 are connected in series.

[0023] Positive output 1014 of 120 VAC source 1004 is connected toground and to one side of 120 VAC output 1006. Negative output 1018 of120 VAC source 1004 is coupled to the other side of 120 VAC output 1006and to one side of 240 VAC output 1008. Switch 1010 switches positiveoutput 1012 of 120 VAC source 1002 and negative output 1016 of 120 VACsource 1002 to switch 120 VAC sources 1002, 1004 between the 120 VACparallel connected mode and the 240/120 VAC series connected mode asdescribed below.

[0024] Switch 1010 is a multi-pole switch, such as a double pole relay,as shown in FIG. 10. When in the parallel connected 120 VAC mode,positive output 1012 of 120 VAC source 1002 is connected to positiveoutput 1014 of 120 VAC source 1004, and thus to ground, by switch 1010and negative outputs 1016, 1018 of sources 1002, 1004, respectively areconnected together by switch 1010. 120 VAC is provided at 120 VAC output1006 by the parallel connected 120 VAC sources 1002, 1004.

[0025] In the 240 VAC series connected mode, positive output 1012 of 120VAC source 1002 is connected through switch 1010 to the other side of240 VAC output 1008, with the first side of 240 VAC output 1008connected to the negative output 1018 of 120 VAC source 1004 asdescribed above. The negative output 1016 of 120 VAC source 1002 isconnected through switch 1010 to ground. 120 VAC is provided at 120 VACoutput 1006 by 120 VAC source 1004 and 240 VAC is provided at 240 VACoutput 1008 by the series connected 120 VAC sources 1002, 1004.

SUMMARY OF THE INVENTION

[0026] A generator system in accordance with the invention has at leasttwo modes of operation where a first output voltage is provided in thefirst mode and the first output voltage and a second output voltage isprovided in the second mode. The second output voltage is twice thefirst output voltage. In an embodiment, the first output voltage isnominally 120 VAC and the second output voltage is nominally 240 VAC.The generator system has a permanent magnet generator with twoindependent sets of windings that each generate a three phase ACvoltage. One three phase AC voltage is coupled to a first or mastercycloconverter and the second three phase AC voltage is coupled to asecond or slave cycloconverter. Live outputs of the cycloconverters arecoupled to each other through a switch, such as a relay, and neutraloutputs of the cycloconverters are coupled to ground. A controllercontrols the cycloconverters to provide the first output voltage,illustratively 120 VAC, across their respective live and neutraloutputs. When in the first mode, such as the 120 VAC mode, the switchacross the live outputs of the first and second cycloconverters isclosed, shorting the live outputs of the first and secondcycloconverters together and the controller operates the first andsecond cycloconverters so that their output voltages are in-phase witheach other. When in the second mode, such as the 240/120 VAC mode, theswitch across the live outputs of the first and second cycloconvertersis open and the controller operates the first and second cycloconvertersso that their output voltages are 180 degrees out of phase. Thisprovides the first output voltage, illustratively 120 VAC, across thelive and neutral outputs of each of the first and second cycloconvertersand the second output voltage that is twice the first output voltage,illustratively 240 VAC, across the live outputs of the first and secondcycloconverters. In an aspect of the invention, the switch is asingle-pole switch such as a single pole relay.

[0027] In an aspect of the invention, the cycloconverters arephase-controlled by the controller and naturally commutated.

[0028] In an aspect of the invention, the permanent magnet generator hasrotor position sensors that sense the position of a rotor of thepermanent magnet generator as it rotates. Outputs of these rotorposition sensors are input to the controller which uses them to generatecontrol wave information that it uses to control the cycloconverters,illustratively cosine control waves

[0029] In an aspect of the invention, the cycloconverters have apositive bank and a negative bank of naturally commutated switchingdevices such as silicon controlled rectifiers (SCRs). In an aspect ofthe invention, each SCR includes an SCR/opto-SCR combination having anSCR and an opto-SCR where the opto-SCR is coupled to a gate of the SCRand used to trigger or control the SCR.

[0030] In an aspect of the invention, voltages across the SCRs of thepositive and negative bank of each of the cycloconverters are sensed andused to determine when the respective cycloconverter bank is in a zerocurrent condition.

[0031] In an aspect of the invention, changeover from a positive to anegative bank of a cycloconverter is initiated by a bandpass filter ofinstantaneous current of the cycloconverter, changeover being initiatedwhen the bandpass filtered instantaneous current transitions about zero,such as by falling within a predetermined range about zero.

[0032] Further areas of applicability of the present invention willbecome apparent from the detailed description provided hereinafter. Itshould be understood that the detailed description and specificexamples, while indicating the preferred embodiment of the invention,are intended for purposes of illustration only and are not intended tolimit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

[0034]FIG. 1 is simplified schematic of a generator system in accordancewith the invention;

[0035]FIG. 2 is a simplified power system diagram of the generatorsystem of FIG. 1;

[0036]FIG. 3 is a timing diagram for control of a cycloconverter of thegenerator system of FIG. 1;

[0037]FIG. 4 is a simplified schematic of circuit logic for initiatingchangeover between a positive and negative bank of a cycloconverter ofthe generator system of FIG. 1;

[0038]FIG. 5 is a simplified schematic of circuit logic for voltagecontrol of the generator system of FIG. 1;

[0039]FIG. 6 is a flow chart showing the development of cosine waveinformation from outputs of rotor position sensors of a permanent magnetgenerator of the generator system of FIG. 1;

[0040]FIG. 7 is a simplified schematic of an SCR/opto-SCR combinationused in a cycloconverter of the generator system of FIG. 1;

[0041]FIG. 8 is a schematic of a voltage sensing circuit that senses thevoltages across the SCRs of a cycloconverter of the generator system ofFIG. 1;

[0042]FIG. 9 is a simplified schematic of a brushless DC motor drivecircuit that can be used in starting the generator system of FIG. 1; and

[0043]FIG. 10 is a simplified schematic of a prior art multi-poleswitching arrangement for switching two 120 VAC sources between paralleland series connected modes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] The following description of the preferred embodiment(s) ismerely exemplary in nature and is in no way intended to limit theinvention, its application, or uses.

[0045] Referring to FIG. 1, a generator system 10, switchable betweenfirst and second modes of operation is shown schematically. In the firstmode, the generator system 10 produces a first output voltage and in thesecond mode, the generator system 10 produces two output voltages, thefirst output voltage and a second output voltage that is twice the firstoutput voltage. In an embodiment, the first output voltage is nominally120 VAC and the second output voltage is 240 VAC and the first mode isthen alternatively referred to as the 120 VAC mode and the second modeis alternatively referred to as the 240/120 VAC mode. In thisembodiment, the first output voltage is referred to as being nominally120 VAC to mean that it is the standard AC voltage used in the UnitedStates for light appliances and devices, such as lamps, power tools,etc. The reference to the second output voltage as being nominally 240VAC is so that it is twice the nominal first output voltage of 120 VAC.

[0046] Generator 10 has an engine 12, illustratively an internalcombustion engine, that drives a generator 14, which is illustratively apermanent magnet generator and which will be referred to herein aspermanent magnet generator 14. Permanent magnet generator 14 has a rotor15 with permanent magnets and a stator with two independent/isolatedsets of three-phase windings 200, 202 (FIG. 2). Permanent magnetgenerator 14 also includes rotor position sensors 16, 18, 20,illustratively hall effect transducers, that sense the position of arotor (not shown) of permanent magnet generator every 120 degreeselectrical. The hall effect transducers are illustratively the halleffect transducers provided as part of permanent magnet generator 14 toenable it to be driven as a brushless DC motor to start engine 12, asdescribed below and as described in Starter System for Portable InternalCombustion Engine Electric Generators Using a Portable Universal BatteryPack, U.S. Ser. No. 60/386,904, filed Jun. 6, 2002, the disclosure ofwhich is incorporated herein in its entirety by reference. Outputs ofthe rotor position sensors 16, 18, 20 are coupled to inputs of acontroller, such as a digital signal processor (DSP) 28.

[0047] Permanent magnet generator 14 generates two separate three-phasevoltages at first set of outputs 30, 32, 34 and second set of outputs36, 38, 40. The outputs 30, 32, 34 at which the first three phasevoltage is generated are coupled to a first (Master) AC power converter42 and the second set of outputs 36, 38, 40 are coupled to a second(Slave) AC power converter 44. In an embodiment of the invention, ACpower converters 42 and 44 are cycloconverters and will be referred toherein as cycloconverters. Each cycloconverter 42, 44 is controlled byDSP 28 to convert the respective three phase voltage coupled to it to anindependent and isolated 120 VAC 60 Hz voltage at their respectiveoutlets 56, 58. It should be understood that a controller other than adigital signal processor can be used, such as a microcontroller. Itshould also be understood that permanent magnet generator 14, DSP 28 andcycloconverters 42, 44 can be configured to produce other voltages andfrequencies, 115 VAC or 50 Hz for example, for markets outside the U.S.,without significant hardware changes. Cycloconverters 42, 44 and theircontrol by DSP 28 will be described in more detail below. Also, in anembodiment, a DSP 28 is provided for each of cycloconverters 42, 44.

[0048] A switch 46, illustratively a single-pole relay, is coupledacross a live output 48 of first cycloconverter 42 and a live output 50of cycloconverter 44. Neutral outputs 52, 54 of first and secondcycloconverters 42, 44, respectively, are coupled to ground. Live output48 of first cycloconverter 42 is coupled to a live output 55 of outlet56 and a neutral output 52 of first cycloconverter 42 is coupled to aneutral output 57 of outlet 56. Live output 54 of second cycloconverter44 is coupled to a live output 59 of outlet 58 and neutral output 50 ofsecond cycloconverter 44 is coupled to a neutral output 61 of outlet 58.

[0049] This configuration provides for two modes of operation forgenerator system 10, 120 VAC and 240/120 VAC in an embodiment of theinvention. In the 120 VAC mode, switch 46 is closed, paralleling thelive outputs 48, 50 of first and second cycloconverters 42, 44,providing increased current output at outlets 56, 58 of generator system10 compared to the 240/120 VAC mode. In the 240/120 VAC mode, first andsecond cycloconverters 42, 44 are controlled by DSP 28 so that thevoltages output by the cycloconverters 42, 44 across their respectivelive outputs 48, 50 to respective neutral outputs 52, 54 are 180 degreesout of phase with each other to enable single point series connectionfor 240 VAC operation. This provides 240 VAC at outlet 60 of generatorsystem 10 and 120 VAC at each of outlets 56, 58. In the 120 VAC mode,first and second cycloconverters 42, 44 are controlled by DSP 28 so thatthe voltages output by the cycloconverters 42, 44 are in-phase with eachother. Having the voltages output by first and second cycloconverters42, 44 in-phase with each other ensures that no circulating currentflows between first and second cycloconverters 42, 44 and thus allowsload sharing between them provided that the output voltages of each offirst and second cycloconverters 42, 44 have the same amplitude. In the120 VAC mode, 120 VAC is provided at the outlets 56, 58 of first andsecond cycloconverters 42, 44, respectively, and outlet 60 is shorted byswitch 46. Operating cycloconverters 42, 44 in this manner allows switch46 to be a single-pole switch, such as a single pole relay.

[0050] It should be understood that this technique of operating the twosources of 120 VAC in phase when they are connected in parallel for the120 VAC mode and 180 degrees out of phase when they are connected inseries for the 240/120 VAC mode can be used with 120 VAC sources havingAC power converters other than cycloconverters, such as (by way ofexample and not of limitation) with inverter circuits or H-Bridgecircuits as disclosed in U.S. Ser. No. 10/0772129 filed Feb. 15, 2002for “Alternator/Inverter with Dual H-Bridge” and in U.S. Ser. No.10/10/077386 filed Feb. 15, 2002 for “Alternator/Inverter with DualH-Bridge and Automatic Voltage Regulation”. The disclosures of these twoapplications are incorporated herein in their entirety by reference.

[0051]FIG. 2 shows in simplified form an overall power system diagram ofgenerator system 10. Permanent magnet generator 14 has, as mentioned,two independent sets of three phase windings, windings 200, 202.Illustratively, permanent magnet generator 14 has a nominal 106 VAC RMSphase voltage (to neutral/star point), 240 Hz electrical at the shaft orrotor, with a phase inductance of 0.7 milli-Henry. It should beunderstood that permanent magnet generator 14 could be configured sothat nominal output values are different. The windings of three phasewindings 200 are identified as A₁, B₁ and C₁ and the windings of threephase windings 202 are identified as A₂, B₂ and C₂. Cycloconverter 42illustratively has two banks of switching devices, positive bank 204 andnegative bank 206 and cycloconverter 44 also illustratively has apositive bank 208 and a negative bank 210 of switching devices. In anaspect of the invention, the switching devices are naturally commutatedswitching devices, such as silicon controlled rectifiers. But it shouldbe understood that other types of naturally commutated switching devicescan be used. Each bank 204, 206, 208, 210 illustratively has six siliconcontrolled rectifiers. The positive and negative banks 204, 206, 208,210 of first and second cycloconverters 42, 44, respectively, form anon-circulating current 6-pulse system. Non-circulating refers to themode of operation where the positive and negative banks of eachcycloconverter 42, 44 do not conduct at the same time. That is, when thecurrent out of a cycloconverter 42, 44 is positive, only the positivebank 204, 208, respectively, of the cycloconverter 42, 44 conducts andwhen the current out of a cycloconverter 42, 44 is negative, only thenegative bank 206, 210, respectively, of the cycloconverter 42, 44conducts. As such, the positive and negative banks 204 and 206 ofcycloconverter 42 are operated so that they do not conduct at the sametime and the positive and negative banks, 208, 210 of cycloconverter 44are also operated so that they do not conduct at the same time.

[0052] Cycloconverters 42, 44 each have an output filter capacitor 212,214 coupled across their respective 120 VAC outputs, shownrepresentatively as resistances 216, 218. The 240 VAC output is shownrepresentatively as resistance 220. Illustratively, filter capacitors212, 214 are 40 microfarad capacitors and 120 VAC outputs 216, 218 ofcycloconverters 42, 44 each have a 3.6 Kw capacity when permanent magnetgenerator 14 has the nominal output values referenced above.

[0053] When generator system 10 is in the 120 VAC mode, DSP 28 controlscycloconverter 42, 44 so that their output voltages are in phase witheach other and when generator system 10 is in the 240/120 VAC mode,cycloconverters 42, 44 are controlled so that their output voltages are180 degrees out of phase. Thus, only the operation of cycloconverter 42will be described. In this regard, the silicon controlled rectifiers ofthe positive bank 204 of cycloconverter 42 are identified as AT+, BT+,CT+, AB+, BB+ and CB+. The silicon controlled rectifiers of the negativebank 206 of cycloconverter 42 are identified as AT−, BT−, CT−, AB−, BB−and CB−.

[0054] Turning to FIGS. 2 and 3, the control of cycloconverter 42 isdescribed. Cycloconverter 42 is controlled by DSP 28 using conventionalcosine control. The inputs to DSP 28 are the three-phase electricaloutputs 30, 32, 34 of windings A₁, B₁ and C₁ of permanent magnetgenerator 14, the back-emf voltage waveforms of the windings of A₁, B₁and C₁ of permanent magnet generator 14, and the output AC voltage andcurrent of cycloconverter 42 as output to filter capacitor 212 and 120VAC output 216, and the load in terms of impedance and power factor.Since the back-emf voltage waveforms of permanent magnet generator 14are not practically measurable, in an aspect of the invention, signalsfrom rotor position sensors 16, 18 and 20 are used by DSP 28 to simulatethe back-emf voltage waveforms and develop the control wave information(illustratively, cosine control waves) for firing control of the siliconcontrolled rectifiers of positive and negative banks 204, 206 ofcycloconverter 42, as described in more detail below. (The termswaveforms and waves are used interchangeably herein. Also, it should beunderstood that a wave may be digital data representative of the wave aswell as an analog signal.)

[0055] Each SCR is controlled in terms of the turn-on instant, butturn-off is controlled by turning on another SCR that reverse biases thefirst SCR, a process known as natural commutation. The SCRs of positivebank 204 can be turned on only when the output current from the SCR'sAT−, BT−, CT−, AT+, BT+ and CT+ is positive. With reference to thetiming diagram of FIG. 3, the three SCRs identified as AT+, BT+ and CT+are fired in a continuing sequence (AT+, BT+, CT+ and starting thesequence again with AT+) by a 3-bit ring counter implemented in DSP 28as long as the output current from the SCR's AT−, BT−, CT−, AT+, BT+ andCT+ of cycloconverter 42 is positive. The three SCRs identified as AB+,BB+ and CB+ are also fired in a perpetual sequence by a second 3-bitring counter implemented in DSP 28 as long as the output current fromthe SCR's AT−, BT−, CT−, AT+, BT+ and CT+ of cycloconverter 42 ispositive. The transitions sequencing the two 3-bit ring countersimplemented in DSP 28 are the comparison points comparing a referencevoltage wave 300 (AT+, BT+, CT+) and an inverse reference voltage wave302 (AB+, BB+, CB+) of reference voltage wave 300, both generated by DSP28, to the corresponding cosine wave of the back-emf voltages ofwindings A₁, B₁ and C₁ of permanent magnet generator 14. For purposes ofclarity, FIG. 3 shows only a full cycle of the cosine wave 304 used tocontrol AT+, which is the cosine wave of the winding A₁ voltage (i.e.,the inverted B₁ voltage), and partial cycles of the cosine waves 306,308 used to control BT+ and CT+. As shown in FIG. 3, when positive bank204 is enabled, AT+ is triggered when the cosine wave 304 becomespositive with respect to reference voltage wave 300. BT+ is thentriggered when the cosine wave 306 becomes positive with respect toreference voltage wave 300. BT+ turning on reverse biases AT+, turningit off. Similarly, CT+ is then triggered when cosine wave 308 becomespositive with respect to reference voltage wave 300 and reverse biasesBT+, turning it off. AB+, BB+ and CB+ are comparably controlled. Theturn-on sequence for one electrical cycle of permanent magnet generator14 where the output current from the SCR's AT−, BT−, CT−, AT+, BT+ andCT+ of cycloconverter 42 is positive is AT+, CB+, BT+, AB+, CT+ and BB+.In this regard, AT+, BT+ and CT+ each stay on until they are reversedbiased by the next AT+, BT+ and CT+ turning on. Each of AB+, BB+ and CB+similarly stay on until they are reverse biased by the next AB+, BB+ andCB+ turning on. Thus, there is always one of AT+, BT+ and CT+ on and oneof AB+, BB+ and CB+ on at the same time when positive bank 204 isenabled.

[0056] Negative bank 206 of cycloconverter 42 is controlled in a similararrangement. The SCRs of negative bank 206 can be turned-on only if theoutput current from the SCR's AT−, BT−, CT−, AT+, BT+ and CT+ ofcycloconverter 42 is negative. The turn-on sequence for one electricalcycle where the output current from the SCR's AT−, BT−, CT−, AT+, BT+and CT+ of cycloconverter 42 is negative is AT−, CB−, BT−, AB−, CT− andBB−.

[0057] Illustratively, comparators are implemented in DSP 28 thatcompare the cosine wave information for the cosine waves, such as cosinewaves 304, 306, 308, to the reference voltage wave information for thereference voltage waves, such as reference voltage waves 300, 302, and,along with the 3-bit ring counters implemented in DSP 28, provide theabove described control of positive and negative banks 204, 206 ofcycloconverter 42. Comparable control of cycloconverter 44 is alsoimplemented in DSP 28.

[0058] As mentioned, signals from rotor position sensors 16, 18 and 20are used by DSP 28 to simulate the back-emf voltage waveforms anddevelop the cosine wave information for firing control of the SCRs ofpositive and negative banks 204, 206 of cycloconverter 42 and also forpositive and negative banks 208, 210 of cycloconverter 44. A typicalbrushless DC motor drive for low speed, high torque applications (usedto start engine 12 of generator system 10 in an aspect of the inventiondescribed below) requires three hall effect transducers installed withinthe motor to sense the position of the rotor. These hall effecttransducers provide on/off logic signals which provide the phaserelationship for the 3-phase excitation of the motor. Normally, eachhall effect transducer provides a transition (e.g., logic 0 to 1) at thezero degree electrical and 180 degree electrical (e.g., logic 1 to 0transition) of the motor for each of the three phase line voltages.Therefore, the output of each hall effect transducer is displaced 120degrees from the outputs of the other two hall effect transducers sothat the three hall effect transducers provide six transitions perrotation of the rotor of permanent magnet generator 14.

[0059] The signals generated by the hall effect transducers thatillustratively are rotor position sensors 16, 18, 20, can directlyrepresent the phasing of the output voltages of permanent magnetgenerator 14 when permanent magnet generator 14 is driven by engine 12.Rotor position sensors 16, 18, 20 will sometimes be referred tohereinafter as hall effect transducers 16, 18, 20. Importantly, thesignals generated by hall effect transducers 16, 18, 20 directlyrepresent the back-emf voltage phasing of permanent magnet generator 14and this information is used to control the commutation of the SCRs incycloconverters 42, 44. Using hall effect transducers 16, 18, 20 in thismanner eliminates the need for filtering with zero phase shift usingterminal voltage information (the voltages at the outputs 30, 32, 34,36, 38, 40) of permanent magnet generator 14, and eliminates the need tocompute the back-emf voltages from the actual terminal voltages ofpermanent magnet generator 14 (i.e., computation of the internal phaseshift of permanent magnet generator 14 caused by load current andinternal reactance). Moreover, since hall effect transducers 16, 18, 20are illustratively the hall effect transducers used for starting engine12 when using a brushless DC motor drive to drive permanent magnetgenerator 14, no additional hall effect transducers are needed.

[0060] Turning to FIG. 6, logic that is illustratively used in DSP 28 todevelop the cosine control wave information for use in controlling thecommutation of the SCRs of cycloconverters 42, 44 is described. Theperiods of each hall effect transducer 16, 18, 20 are measured (usingrising edges only) at 600, 602, 604 to establish a period (“Hallperiod”). This Hall period is updated on each rising edge of a halleffect transducer 16, 18, 20. At 606, the Hall period is divided by 84to establish a period for a timer so that the timer will have 84overflows per Hall period, i.e., time out 84 times per Hall period. At608, each timer overflow resets the timer to zero and increments acounter (“Hall period counter”) by one.

[0061] The value of the Hall period counter, the value of the Hallperiod counter plus an offset of 28 (120 degrees electrical) and thevalue of the Hall period counter plus an offset of 56 (240 degreeselectrical) are then used as pointers into a sine wave look-up tablehaving 84 entries. The three entries in this sine wave look-up tablepointed to by these pointers are read from the sine wave look-up tableat 616 and at 618 these three values become the cosine values that arecompared to the reference voltage wave, such as reference voltage wave300, to control the commutation of the SCRs of cycloconverter 42.Cycloconverter 44 is similarly controlled.

[0062] With reference to FIG. 4, bank selection and changeover controlin accordance with an aspect of the invention is described. Asmentioned, for each cycloconverter 42, 44, only its positive bank 204,208 or negative bank 206, 210 can be on at any given time. Bankselection and changeover control of cycloconverters 42, 44 is doneidentically, so it will be described with reference to cycloconverter42.

[0063] Assuming that the output voltage at output 216 of cycloconverter42 is a sine wave with a frequency of 60 Hz, positive and negative banks204, 206 are each on for one-half period of the 60 Hz cycle. Which ofthe positive and negative banks 204, 206 that is conducting isdetermined by the polarity of the output current from the SCR's AT−,BT−, CT−, AT+, BT+ and CT+, which for the purposes of this discussion isassumed to be at the same 60 Hz frequency but with a power factordependent on load.

[0064] The selection of positive and negative banks 204, 206 (i.e.,which one is enabled so that its SCRs can be triggered to conduct andwhich one is disabled so that its SCRs cannot be triggered to conduct)is determined from the measured instantaneous output current ofcycloconverter 42 to filter capacitor 212 and output 216. Thisinstantaneous current is filtered, illustratively by bandpass filter400, to eliminate current ripple and to ensure that the fundamental 60Hz component of the signal output by bandpass filter 400 does not haveany phase-shift relative to the instantaneous current. Illustratively,bandpass filter 400 is a 2-pole 60 Hz bandpass filter having a Q of 2.The filtered current signal output from bandpass filter 400 is theninput to a comparator 402. Comparator 402 is illustratively a hysteresiscomparator, illustratively having a negative hysteresis with switchinglevels of either +0.1 A or −0.1 A. The output of comparator 402determines whether the fundamental 60 Hz current to filter capacitor 212and output 216 of cycloconverter 42 is positive or negative. Comparator402 switches from positive to negative when the filtered current signaloutput by bandpass filter 400 drops below +0.1 A and switches fromnegative to positive when the filter current signal output by bandpassfilter 400 increases above −0.1 A. When comparator 402 switches frompositive to negative, DSP 28 disables positive bank 204 ofcycloconverter 42 and, following a delay of 100 microseconds after anactual output current zero is detected, enables negative bank 206 ofcycloconverter 42. As such, no more trigger pulses are fed to the gateterminals of the SCRs in positive bank 204. However, the SCRs in thepositive bank 204 that are conducting when this transition occurs willcontinue to conduct until a true current zero occurs. At this point,they will be reversed biased and turn off. Conversely, when comparator402 switches from negative to positive, DSP 28 disables negative bank206 of cycloconverter 42, and, following a delay of 100 microsecondsafter an actual output current zero is detected, enables positive bank204. As was the case with positive bank 204, the SCRs in negative bank206 that are conducting when this transition occurs will continue toconduct until a true current zero occurs, at which time they arereversed biased and turn off.

[0065] The true current zero condition may be sensed by comparator 404.Comparator 404 is illustratively a two-window comparator that determineswhen the actual output current (not the filtered fundamental current)drops within a window of +/−25 mA. Comparator 404 illustratively hasfirst and second comparators 406, 408 having their outputs coupled toinputs of an AND gate 410. A positive input of comparator 406 is coupledto a +25 mA reference and a negative input of comparator 408 coupled toa −25 mA reference. The current output to filter capacitor 212 andoutput 216 of cycloconverter is coupled to a negative input ofcomparator 406 and to a positive input of comparator 408. When theoutput current falls within the +/−25 mA window, the outputs of bothcomparators 406, 408 will be positive, resulting in the output of ANDgate 410 being positive, indicating a true current zero. In an aspect ofthe invention described below, the true current zero condition is sensedindirectly by sensing the voltages across the SCRs of cycloconverters42, 44.

[0066] Once this true zero current condition is detected, a delay isimposed, illustratively, 100 microseconds, to ensure that the SCRspresently conducting have enough time to turn off. After this delay,change over from positive bank 204 to negative bank 206 (or vice-versa)occurs.

[0067] It should be understood that the above bank changeover controllogic is illustratively implemented in DSP 28. However, it should alsobe understood that all or portions of the above bank changeover controlcould be implemented using discrete components, such as using voltagesensing circuit 800 to indirectly determine the true current zerocondition, as described below.

[0068] For a generator system, the output voltage is a sinusoidalwaveform having the voltage and frequency required by of the countrywhere it is used, for example, 120 VAC, 60 Hz in the United States. Ascaled equivalent(s) of this waveform, for example, waves 300, 302 inFIG. 3, is used in the control of the SCRs of cycloconverter 42 (andcycloconverter 44). However, some form of output voltage control isneeded owing to the fact that the voltage generated by generator system10 (FIG. 1) is directly proportional to the speed at which the rotor ofpermanent magnet generator 14 is spinning and that the load on permanentmagnet generator 14 causes a voltage drop across a phase reactance ofpermanent magnet generator 14.

[0069]FIG. 5 shows an illustrative voltage control implemented in DSP28. The instantaneous output voltage (Vout) of generator system 10 ismeasured and input into DSP 28. The absolute value of Vout is thenfiltered and, after being suitably scaled, is used as the feedback termin a proportional feedback loop. The filter that filters the absolutevalue of Vout is illustratively a 2-pole low pass filter with a cut-offfrequency of 3.2 Hz and a Q of 0.25. The output of this filter is anaverage value of the absolute value of Vout with the 60 Hz/120 Hzcomponents removed. The scaling factor used to scale the output of thisfilter is illustratively 0.00926 such that a value of 1.0 corresponds toa sinusoidal AC output voltage of 120 VAC RMS.

[0070] The input reference, Vref, to the feedback loop is the averagevalue of the reference voltage wave generated by DSP 28 and is assigneda value of 1.0, such as reference voltage wave 300 (FIG. 3), whichcorresponds to a 120 VAC 60 Hz sine wave. The output (Vc) of theproportional feedback loop is used to directly control the effectivemagnitude of the cosine waves for SCR firing control, such as cosinewaves 304, 306, 308 (FIG. 3). The proportional gain used is 16, where aproportional output at Vc of 1.0 produces the maximum output voltage atthe output of cycloconverters 42, 44. A proportional output value at Vcof 2.0 produces one-half the maximum output voltage at the output ofcycloconverter 42. Therefore, the proportional feedback loop of FIG. 5is set up such that as the output voltage of cycloconverter 42 dropsbelow 120 VAC, the output of the proportional gain stage of theproportional feedback loop of FIG. 5 reduces, causing the output voltageof cycloconverter 42 to increase, providing output voltage control forgenerator system 10. Comparable control is provided for cycloconverter44.

[0071] Referring to FIGS. 1 and 2, as discussed, switch 46 isillustratively a relay and is illustratively controlled by a switch 62on a front panel (not shown) of generator system 10. Switch 62illustratively provides an input signal to DSP 28 that in turn controlsswitch 46.

[0072] Operating cycloconverters 42, 44 in series or parallel doesn'tpresent any particular problems. However, the transition between seriesand parallel operation requires careful timing control.

[0073] DSP 28 controls the transition of generator system 10 betweenseries (240/120 VAC) and parallel (120 VAC) operation. When switch 62 isthrown to switch generator system 10 from series (240/120 VAC) operationto parallel (120 VAC) operation, the outputs of one of first and secondcycloconverters 42, 44 are disabled (i.e., all its SCRs are no longerturned on). After an appropriate delay, illustratively 3.5 electrical 60Hz or 50 Hz cycles, that cycloconverter 42, 44 is re-enabled in phasewith respect to the output voltage of the other cycloconverter 42, 44.Switch 46 is then closed.

[0074] The transition from parallel (120 VAC) to series (240/120 VAC)operation is controlled by DSP 28 in similar fashion. When switch 62 isthrown to switch generator system 10 from parallel to series operation,switch 46 is immediately opened. The outputs of first and secondcycloconverters 42, 44 are both disabled (all their SCRs are no longerturned on). After an appropriate delay, illustratively 3.5 electrical 60Hz or 50 Hz cycles, first and second cycloconverters are re-enabled with180 degrees phase shift between their outputs.

[0075] In an embodiment of the invention, a DSP 28 is provided tocontrol cycloconverter 42 and a second DSP 28 is provided to controlcycloconverter 44 with the two DSPs 28 linked via a high speed 2-wayisolated serial communication link to handle the control between thefirst and second cycloconverters 42, 44. Illustratively, the DSP 28 forfirst cycloconverter 42 defines the phasing of the 60 Hz output waveformto the DSP 28 for the second cycloconverter 44 and also provides outputvoltage and current measurement information to the DSP 28 for the secondcycloconverter 44 to keep first cycloconverter 42 and secondcycloconverter 44 synchronized. Each DSP 28 may illustratively be aTMS320LC2402A available from Texas Instruments, Inc. of Dallas, Tex.

[0076] In aspect of the invention, the true instantaneous zero currentcondition at the output of each cycloconverter 42, 44 may be detectedindirectly by monitoring the three phase voltages output by permanentmagnet generator 14 to the first and second outputs of the respectivecycloconverters 42, 44. This is done identically for bothcycloconverters 42, 44, so it will be described with reference tocycloconverter 42. The voltages from the outputs 30, 32, 34, ofpermanent magnet generator 14 to the live and neutral outputs 48, 52 ofcycloconverter 42 are sensed by sensing the voltages across each of theSCRs of the positive and negative banks 204, 206 of cycloconverter 42.When the voltage across any of the SCRs of cycloconverter 42 is lessthan a certain absolute value, such as +8 to 9 volts, this means thatthe SCR is conducting and the output current of the cycloconverter ofwhich that SCR is part is not zero. When there is more than +8 to +9volts or less than −8 to −9 volts across all the SCRs of cycloconverters42, it means that all the SCRs of cycloconverter 42 are blocking and theoutput of cycloconverter 42 is at the zero current condition. This zerocurrent condition is processed into a digital signal and input into DSP28 where it is used for bank changeover control as discussed above. FIG.8 is a schematic of such a voltage sensing circuit 800 that senses thevoltages across the SCRs of cycloconverter 42. Voltage sensing circuit800 includes voltage sensing circuit 802 that senses the voltages acrossthe SCRs identified as AT+, AT−, BT+, BT−, CT+ and CT− of positive andnegative banks 204, 206 of cycloconverter 42 and voltage sensing circuit804 that senses the voltages across the SCRs identified as AB+, AB−,BB+, BB−, CB+ and CB− of positive and negative banks 204, 206 ofcycloconverter 42.

[0077] In an aspect of the invention, the SCRs of positive and negativebanks 204, 206 of cycloconverter 42 and 208, 210 of cycloconverter 44are illustratively SCR/opto-SCR combinations. With reference to FIG. 7,a SCR/opto-SCR combination 700 is shown for the SCR of positive bank 204of cycloconverter 42 identified as AT+. SCR/opto-SCR combination 700 hasan SCR 702 having its anode coupled to output 30 of permanent magnetgenerator 14 (FIG. 1) and its cathode coupled to output 48 ofcycloconverter 42 (FIG. 1). A gate of SCR 702 is coupled to the cathodeof an opto-SCR 704. An anode of opto-SCR 704 is coupled through aresistor 706 to the anode of SCR 702. A gate light emitting diode 708 ofopto-SCR 704 is coupled to an output of DSP 28. SCR 702 isillustratively a S6016R available from Teccor Electronics of Irving,Tex., and opto-SCR 704 is illustratively a TLP741J available fromToshiba America Electronic Components, Inc. of Irvine, Calif.

[0078]FIG. 9 is a simplified schematic drawing of an aspect of theinvention where a brushless DC drive circuit 900 is used in combinationwith generator system 10 (FIG. 1) to drive permanent magnet generator 14for starting engine 12 (FIG. 1), similar to that which is described inthe above referenced U.S. Ser. No. 60/077219 “Starter System forPortable Internal Combustion Engine electric Generators Using a PortableUniversal Battery Pack.” Circuit 900 is a low voltage DC to AC 3-phaseinverter that incorporates a Brushless DC/Permanent magnet generator(BLDC/PMG) starter control 902, and is powered directly by a battery,illustratively, a universal battery pack 903, such as a universalbattery pack from the DEWALT XR PLUS (Extended Run Time) universalbattery pack line. DC drive circuit 900 includes a power stage 904 thatis electrically connectable to permanent magnet generator 14 through a3-pole relay switch 906. Power stage 904 includes six identical powerswitching devices 908 a-908 f coupled across DC bus lines, or rails, 910and 912. Power switching devices 908 a and 908 b are connected in seriesbetween bus lines 910 and 912 having a center node 914 electricallyconnected to one pole of relay 906. Power switching devices 908 c and908 d are connected in series between bus lines 910 and 912 having acenter node 916 electrically connected to a second pole of relay 906.Power switching devices 908 e and 908 f are similarly connected inseries between bus lines 910 and 912 having a center node 918electrically connected to a third pole of relay 906. Six diodes 920a-920 f are respectively connected in parallel with switching devices908 a-908 f, between bus lines 910 and 912. Switching devices 908 a-908f may comprise a variety of suitable power switching components, forexample field effect transistors (FET's), insulated gate bi-polartransistors (IGBTs), or metal oxide silicon field effect transistors(MOSFET's).

[0079] The hall effect transducers 16, 18, 20 of permanent magnetgenerator 14 are connected to inputs of BLDC/PMG starter control 902.Additionally, DC drive circuit 250 includes a momentary starter switch922 that controls the flow of current from universal battery pack 903 toBLDC/PMG starter control 902.

[0080] In operation, engine 12 is initially at rest. Engine 12 isstarted by a user closing momentary start switch 922. The BLDC/PMGstarter control 902 will then become energized by universal battery pack903. Provided the hall effect transducers 16, 18, 20 indicate thateither the speed of engine 12 or the speed of permanent magnet generator14 is less than a predetermined value, e.g. 500 rpm, 3-pole relay switch906 will be energized by BLDC/PMG starter control 902, therebyconnecting the 3-phase power stage 904 to permanent magnet generator 14.Utilizing information from hall effect transducers 16, 18, 20, BLDC/PMGstarter control 902 turns the switching devices 908 a-908 f on and offto provide torque to engine 12 using electronic commutation of the firstset 200 of 3-phase windings (or a tapped winding from such) withinpermanent magnet generator 14. Engine 12 will be turned by permanentmagnet generator 14, driven as a motor in a “Motor Mode” by power stage904 under control of BLDC/PMG starter control 902, to accelerate engine12 to a speed at which engine 12 starts. Once engine 12 has started,permanent magnet generator 14 is driven past a predetermined maximumspeed, e.g. 500 rpm, and 3-pole relay switch 906 will then bede-energized by BLDC/PMG starter control 902, thereby disconnectingpower stage 904 from permanent magnet generator 14. Disconnecting powerstage 904 avoids overdriving universal battery pack 903 and supplyingexcessive voltage to switching devices 908 a-908 f. Once the startingoperation is complete, momentary start switch 922 is opened and BLDC/PMGstarter control 902 ceases turning switching devices 908 a-908 f on andoff.

[0081] BLDC/PMG starter control 902 can be microprocessor based tosimplify the electronic circuitry and to provide additional controlfeatures. Additional control features may include setting a maximumcranking time, e.g. five seconds, to avoid damage if momentary startswitch 922 is held closed for too long, or not allowing starting whenuniversal battery pack 903 does not have sufficient voltage to turn orstart engine 12. Further control features provided by a microprocessorbased BLDC/PMG starter control 902 include speed detection and controlof 3-pole relay switch 906 to avoid overdriving universal battery pack903 and power stage 904, or setting an upper starting speed of permanentmagnet generator 14 regardless of the voltage of universal battery pack903 by utilizing pulse width modulation control of switching devices 908a-908 f above a minimum speed.

[0082] The description of the invention is merely exemplary in natureand, thus, variations that do not depart from the gist of the inventionare intended to be within the scope of the invention. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention.

What is claimed is:
 1. A generator system having at least first andsecond modes, the generator system producing a first alternating currentoutput voltage when in the first mode and producing the firstalternating current output voltage and a second alternating currentoutput voltage when in the second mode, the second output voltage beingtwice the first output voltage, comprising: first and second voltagesources each having an output at which they produce the first outputvoltage; a switch coupling the first and second outputs of the first andsecond voltage sources in parallel when the switch is in a firstposition and in series when the switch is in a second position, thefirst output voltage produced at the outputs of the first and secondvoltage sources when the switch is in the first position and the secondoutput voltage produced across the series coupled outputs of the firstand second voltage sources when the switch is in the second positionwith the first output voltage also produced at the outputs of the firstand second voltage sources when the switch is in the second position. 2.The generator system of claim 1 wherein the current available at thefirst output voltage when the outputs of first and second voltage sourceare coupled in parallel is greater than the current available at thefirst output voltage when the outputs of the first and second voltagesources are coupled in series.
 3. The generator system of claim 1 andfurther including a controller coupled to the first and second voltagesources, the controller operating the first and second voltage sourcesso that their first output voltages are in phase when the switch is inthe first position and one-hundred and eighty degrees out of phase whenthe switch is in the second position.
 4. The generator system of claim 3wherein the controller includes a first controller for controlling thefirst voltage source and a second controller for controlling the secondvoltage source.
 5. The generator system of claim 1 wherein the first andsecond voltage sources each include a generator coupled to an AC powerconverter having an output that provides the output of that first andsecond voltage source.
 6. The generator system of claim 5 wherein asingle generator having first and second sets of windings provides thegenerators of the first and second voltage sources, the first set ofwindings coupled to the AC power converter of the first voltage sourceto provide the generator of the first voltage source and the second setof windings coupled to the AC power converter of the second voltagesource to provide the generator of the second voltage source.
 7. Thegenerator system of claim 6 wherein the AC power converters of the firstand second voltage sources include cycloconverters.
 8. The generatorsystem of claim 7 and further including a controller coupled to thecycloconverters of the first and second voltage sources, the controlleroperating the cycloconverters of the first and second voltage sources sothat they are in phase when the switch is in the first position andone-hundred and eighty degrees out of phase when the switch is in thesecond position.
 9. The generator system of claim 8 wherein thecontroller operates the cycloconverters using cosine control.
 10. Thegenerator system of claim 9 wherein the generator includes at least onerotor position sensor that senses the position of a rotor of thegenerator and generates a signal indicative of the position of therotor.
 11. The generator system of claim 10 wherein the controller usesthe rotor position signal to develop control waves which it uses tocontrol the cycloconverters.
 12. The generator system of claim 9 whereinthe generator includes rotor position sensors that generate signalsindicative of the position of the rotor that are displaced one-hundredand twenty degrees from each other.
 13. The generator system of claim 12wherein the controller uses the rotor position signals to developcontrol waves which it uses to control the cycloconverters.
 14. Thegenerator system of claim 13 wherein each cycloconverter includes apositive and a negative bank of naturally commutated switching devices.15. The generator system of claim 14 wherein the controller generates areference wave and controls the cycloconverters by generating firingsignals for the naturally commutated switching devices based oncomparisons of the control waves to the reference wave.
 16. Thegenerator system of claim 15 wherein the naturally commutated switchingdevices include silicon controlled rectifiers.
 17. The generator systemof claim 15 wherein each naturally commutated switching devices includesa silicon controlled rectifier/opto-silicon controlled rectifiercombination.
 18. The generator system of claim 15 wherein the first andsecond generators are three-phase generators with each of the first andsecond sets of windings having at least one winding for each phase. 19.The generator system of claim 10 wherein the generator includes anengine and a brushless DC motor drive circuit coupled to at least oneset of the generator windings for driving the generator as a brushlessDC motor to start the engine, the rotor position sensor coupled to abrushless DC motor controller of the brushless DC motor drive circuit.20. The generator system of claim 12 wherein the generator includes anengine and a brushless DC motor drive circuit coupled to at least oneset of the generator windings for driving the generator as a brushlessDC motor to start the engine, the rotor position sensors coupled to abrushless DC motor controller of the brushless DC motor drive circuit.21. The generator system of claim 10 wherein the rotor position sensorincludes a hall effect transducer.
 22. The generator system of claim 12wherein the rotor position sensors include hall effect transducers. 23.The generator system of claim 20 wherein the rotor position sensorsinclude hall effect transducers.
 24. The generator system of claim 1wherein the switch includes a single pole switch.
 25. The generatorsystem of claim 24 wherein the outputs of the first and second voltagesources each have live and neutral outputs, the single pole switch beingone single pole relay with the single pole of the single pole relaycoupled across the live output of the first voltage source and the liveoutput of the second voltage source.
 26. The generator system of claim14 wherein the controller operates the positive and negative banks ofnaturally commutated switching devices of each cycloconverter in anon-circulating mode, the controller enabling one of the positive andnegative banks and disabling the other of the positive and negativebanks of each cycloconverter based on the instantaneous output currentof that cycloconverter.
 27. The generator system of claim 26 wherein thecontroller disables the positive bank of each of the cycloconverterswhen the instantaneous output current of that cycloconverter transitionsfrom positive to negative, and then enables the negative bank of thatcycloconverter only after a true zero current condition at the output ofthat cycloconverter occurs, the controller further disabling thenegative bank of each of the cycloconverters when the instantaneousoutput current of that cycloconverter transitions from negative topositive and then enables the negative bank of that cycloconverter onlyafter a true zero current condition at the output of that cycloconverteroccurs.
 28. The generator system of claim 27 wherein for eachcycloconverter the true zero current condition at the output of thatcycloconverter is determined by a comparator determining that actualoutput current of that cycloconverter is between first and secondreference levels.
 29. The generator system of claim 28 wherein the firstand second reference levels are +25 mA and −25 mA.
 30. The generatorsystem of claim 27 wherein for each cycloconverter the controllerdetermines that the true zero current condition at the output of thatcycloconverter occurs by sensing that the positive and negative banks ofthat cycloconverter are non-conducting.
 31. The generator system ofclaim 30 wherein the controller sensing that the positive and negativebanks of one of the cycloconverters are non-conducting includes sensingthat the voltage across each of the naturally commutated switchingdevices of the positive and negative banks of that cycloconverter isabove a predetermined level.
 32. The generator system of claim 27 andfurther including a bandpass filter for each cycloconverter forfiltering the instantaneous output current of that cycloconverter toreduce current ripple and ensure that a signal output by the bandpassfilter at a fundamental frequency does not have any phase-shift relativeto the instantaneous output current of that cycloconverter, the signaloutput by the bandpass filter coupled to an input of a comparator thatgenerates a signal indicative of whether the instantaneous outputcurrent transitioned from positive to negative or from negative topositive.
 33. The generator system of claim 32 wherein the fundamentalfrequency is 60 Hz.
 34. The generator system of claim 1 wherein thefirst output voltage is nominally 120 VAC and the second output voltageis nominally 240 VAC.
 35. A generator system, comprising: an ACgenerator having an output coupled to a cycloconverter, thecycloconverter having a positive bank of naturally commutated switchingdevices and a negative bank of naturally commutated switching devices; acontroller coupled to the naturally commutated switching devices of thepositive and negative banks; a rotor position sensor for sensing theposition of a rotor of the generator and generating a signal indicativeof the position of the rotor, the rotor position sensor coupled to thecontroller; the controller using the rotor position signal to developcontrol waves which it uses to control switching of the naturallycommutated switching devices of the positive and negative banks.
 36. Thegenerator system of claim 35 wherein the rotor position sensor includesa plurality of rotor position sensors that generate signals indicativeof the position of the rotor that are displaced one-hundred and twentydegrees from each other.
 37. The generator system of claim 36 whereinthe plurality of rotor position sensors include hall effect transducers.38. The generator system of claim 36 wherein the controller generates areference wave and generates firing signals for the naturally commutatedswitching devices of the positive and negative banks based oncomparisons of the control waves to the reference wave.
 39. Thegenerator system of claim 38 wherein the AC generator includes an engineand a brushless DC motor drive circuit coupled to the AC generator fordriving the AC generator as a brushless DC motor to start the engine,the rotor position sensors coupled to a brushless DC motor controller ofthe brushless DC motor drive circuit.
 40. The generator system of claim36 wherein for each cycloconverter the controller operates the positiveand negative banks of that cycloconverter in a non-circulating mode, thecontroller enabling one of the positive and negative banks and disablingthe other of the positive and negative banks based on the instantaneousoutput current of that cycloconverter.
 41. The generator system of claim40 wherein, for each cycloconverter, the controller enables one of thepositive and negative banks of that cycloconverter and disables theother of the positive and negative banks of that cycloconverter based onthe instantaneous output current of that cycloconverter transitioningbetween positive and negative or between negative and positive whereinthe controller enables one of the positive and negative banks of thatcycloconverter only after a true current zero condition occurs at theoutput of that cycloconverter.
 42. The generator system of claim 41wherein for each cycloconverter the true zero current condition at theoutput of that cycloconverter is determined by a comparator determiningthat actual current output current of that cycloconverter falls betweenfirst and second reference levels.
 43. The generator system of claim 42wherein the first and second reference levels are +25 mA and −25 mA. 44.The generator system of claim 41 wherein the controller determines thatthe true zero current condition at the output of one of thecycloconverters occurs by sensing that the positive and negative banksare non-conducting.
 45. The generator system of claim 44 wherein thecontroller sensing that the positive and negative banks arenon-conducting includes sensing that that voltage across each of thenaturally commutated switching devices and sensing that the positive andnegative banks is above a predetermined level.
 46. The generator systemof claim 41 and further including a bandpass filter for filtering theinstantaneous output current of the cycloconverter to reduce currentripple and ensure that a signal output by the bandpass filter at afundamental frequency does not have any phase-shift relative to theinstantaneous output current, the signal output by the bandpass filtercoupled to an input of a comparator that generates a signal indicativeof whether the instantaneous current output has transitioned frompositive to negative or from negative to positive.
 47. The generatorsystem of claim 46 wherein the fundamental frequency is 60 Hz.
 48. Thegenerator system of claim 35 wherein the naturally commutated switchingdevices include silicon controlled rectifiers.
 49. The generator systemof claim 35 wherein each naturally commutated switching device includesa silicon controlled rectifier/opto-silicon controlled rectifiercombination.
 50. A generator system, comprising: an AC generator havingan output coupled to a cycloconverter, the cycloconverter having apositive bank of naturally commutated switching devices and a negativebank of naturally commutated switching devices; a controller coupled tothe naturally commutated switching devices of the positive and negativebanks; the controller operating the positive and negative banks in anon-circulating mode, the controller enabling one of the positive andnegative banks and disabling the other of the positive and negativebanks based on the instantaneous output current of the cycloconverter; abandpass filter for filtering the instantaneous output current of thecycloconverter to reduce current ripple and ensure that a signal outputby the bandpass filter at a fundamental frequency does not have anyphase shift relative to the instantaneous output current, the signaloutput by the band pass filter coupled to an input of a comparator thatgenerates a signal indicative of whether the instantaneous outputcurrent has transitioned from positive to negative or from negative topositive.
 51. The generator system of claim 50 wherein the controllerenables one of the positive and negative banks only after it disablesthe other of the positive and negative banks and it senses that anoutput of the cycloconverter has passed through a true zero currentcondition, the controller sensing that the true zero current conditionat the output of the cycloconverter occurs when a voltage across each ofthe naturally commutated switching devices is above a predeterminedlevel indicating that each of the naturally commutated switching devicesis non-conducting.
 52. The generator system of claim 50 wherein thefundamental frequency is 60 Hz.
 53. The generator system of claim 50wherein each naturally commutated switching device includes a siliconcontrolled rectifier/opto-silicon controlled rectifier combination. 54.A generator system, comprising: an AC generator having an output coupledto a cycloconverter, the cycloconverter having a positive bank ofnaturally commutated switching devices and a negative bank of naturallycommutated switching devices; each naturally commutated switching deviceincluding a silicon controlled rectifier/opto-silicon controlledrectifier combination.
 55. A dual mode generator system having a nominaloutput voltage of 120 VAC when it is in a first mode and nominal outputvoltages of both 120 VAC and 240 VAC when it is in a second mode,comprising: a permanent magnet generator having first and second sets ofisolated three-phase windings and a rotor; an engine for driving therotor of the permanent magnet generator; a plurality of rotor positionsensors that generate signals indicative of the position of the rotorthat are displaced one-hundred and twenty degrees from each other; afirst cycloconverter coupled to the first set of windings of thegenerator and a second cycloconverter coupled to the second set ofwindings of the generator; the first and second cycloconverters eachhaving a live output and a neutral output, the live output of the firstcycloconverter coupled to a live output of a first outlet and theneutral output of the first cycloconverter coupled to a neutral outputof the first outlet, the live output of the second cycloconvertercoupled to a live output of a second outlet and the neutral output ofthe second cycloconverter coupled to a neutral output of the secondoutlet; the first and second cycloconverters each having a positive bankof naturally commutated switching devices and a negative bank ofnaturally commutated switching devices; a first filter capacitor coupledacross the live output and neutral output of the first cycloconverterand a second filter capacitor coupled across the live output and neutraloutput of the second cycloconverter; a third outlet coupled across thelive outputs of the first and second outlets and a switch coupled acrossthe live outputs of the first and second outlets; a controller coupledto the naturally commutated switching devices and to the rotor positionsensors; the controller using the signals generated by the rotorposition sensors to develop cosine control waves which it uses tocontrol the switching of the naturally commutated switching devices ofthe first and second cycloconverters; the controller operating the firstand second cycloconverters so that output voltages at the live outputsof the first and second cycloconverters are in phase when the generatorsystem is in a first mode with the switch closed paralleling the liveoutputs of the first and second cycloconverters where the nominal 120VAC output voltage is produced at the first and second outlets with agreater available current than when the generator system is in thesecond mode; the controller operating the first and secondcycloconverters so that the output voltages at the live outputs of thefirst and second cycloconverters are one-hundred and eighty degrees outof phase with each other when the generator system is in a second modewith the switch open coupling the third outlet in series with the liveoutputs of the first and second cycloconverters with the nominal 120 VACoutput voltage produced at the first and second outlets and the nominal240 VAC output voltage produced at the third outlet.
 56. The generatorsystem of claim 55 wherein and further including an engine to drive thegenerator and a brushless DC motor drive circuit coupled to thegenerator and the rotor position sensors, the brushless DC motor drivecircuit driving the generator as a brushless DC motor to start theengine.
 57. The generator system of claim 56 wherein the rotor positionsensors include hall effect transducers.
 58. The generator system ofclaim 55 wherein the switch is a single pole switch.
 59. The generatorsystem of claim 58 wherein the single pole switch is a single polerelay.
 60. The generator system of claim 55 wherein each naturallycommutated switching device includes a silicon controlledrectifier/opto-silicon controlled rectifier combination.
 61. Thegenerator system of claim 55 wherein the controller operates thepositive and negative banks of naturally commutated switching devices ofeach cycloconverter in a non-circulating mode, the controller enablingone of the positive and negative banks of each cycloconverter anddisabling the other of the positive and negative banks of eachcycloconverter based on the instantaneous output current of thatcycloconverter transitioning between positive and negative or betweennegative and positive wherein the controller enables one of the positiveand negative banks of each cycloconverter only after it senses that atrue current zero condition occurs at the live output of thatcycloconverter.
 62. The generator system of claim 61 wherein thecontroller senses that a true current zero condition occurred at a liveoutput of one of the cycloconverters when it senses that a voltageacross each of the naturally commutated switching devices of thepositive and negative banks of that cycloconverter is above apredetermined level indicating that each of the naturally commutatedswitching devices of that cycloconverter is non-conducting.
 63. Thegenerator system of claim 62 and further including a bandpass filter foreach cycloconverter for filtering the instantaneous output current atthe live output of the cycloconverter to reduce current ripple andensure that a fundamental 60 Hz component of a signal output by eachbandpass filter does not have any phase-shift relative to theinstantaneous output current at the live output of the cycloconverter,the signal output by the bandpass filter input to a comparator thatgenerates a signal indicative of whether the instantaneous outputcurrent transitions between positive and negative or between negativeand positive.
 64. The generator system of claim 55 wherein when thegenerator system switches between modes, the controller, if thegenerator system is switching from the first mode to the second mode: a.opens the switch; b. then disables the naturally commutated switchingdevices of the first and second cycloconverters so that they are allnon-conducting; and c. after a predetermined delay, reenables thenaturally commutated switching devices of the first and secondcycloconverters so that the live outputs of the first and secondcycloconverters are one-hundred and eighty degrees out of phase witheach other; and the controller if the generator system is switching fromthe second mode to the first mode: a. disables the naturally commutatedswitching devices of one of the first and second cycloconverters; b.reenables after a predetermined delay the naturally commutated switchingdevices that were disabled so that the live outputs of the first andsecond cycloconverters are in-phase; and then c. closes the switch. 65.The generator system of claim 64 wherein the predetermined delay is 3.5electrical cycles.
 66. A method of controlling a generator system havingat least first and second modes where the generator system produces afirst alternating current output voltage when it is in the first modeand produces the first output voltage and a second alternating currentoutput voltage when it is in the second mode, the second output voltagetwice the first output voltage, comprising: coupling the outputs of thefirst and second voltage sources in parallel and operating the first andsecond voltage sources so that the voltages at the outputs of the firstand second voltage sources are in phase when the generator system is inthe first mode; and coupling the outputs of the first and second voltagesources in series and operating the first and second voltage sources sothat the voltages at the outputs of the first and second voltage sourcesare one-hundred and eighty degrees out of phase when the generatorsystem is in the second mode with the second output voltage producedacross the series coupled outputs of the first and second voltagesources and the first output voltage produced at each of the outputs ofthe first and second voltage sources.
 67. The method of claim 66including generating rotor position signals indicative of a position ofa rotor of a permanent magnet generator and developing control wavesfrom the rotor position signals, and using the control waves to controlfirst and second cycloconverters, the first cycloconverter coupled to afirst set of windings of the permanent magnet generator and the secondcycloconverter coupled to a second set of windings of the permanentmagnet generator, the first cycloconverter having an output thatprovides the output of the first voltage source and the secondcycloconverter having an output that provides the output of the secondvoltage source.
 68. The method of claim 67 including generating areference wave, comparing the control waves to the reference wave, andcontrolling switching of naturally commutated switching devices of thefirst and second cycloconverters based on the comparisons of the controlwaves to the reference wave.
 69. The method of claim 67 wherein thegenerator system includes an engine for driving the permanent magnetgenerator, the method including driving at least one of the first andsecond sets of windings of the permanent magnet generator with abrushless DC motor drive circuit to drive the permanent magnet generatoras a brushless DC motor to start the engine, a brushless DC motorcontroller of the brushless DC motor drive circuit using the rotorposition signals in driving the permanent magnet generator as abrushless DC motor.
 70. The method of claim 67 wherein eachcycloconverter includes a positive and a negative bank of naturallycommutated switching devices, the method including operating thepositive and negative banks of each cycloconverter in a non-circulatingmode and enabling one of the positive and negative banks and disablingthe other of the positive and negative banks of each cycloconverterbased on the instantaneous output current of that cycloconvertertransitioning between positive and negative or between negative andpositive wherein the one of the positive and negative banks of one ofthe cycloconverter that is being enabled is enabled only after a truecurrent zero condition occurs at the output of that cycloconverter. 71.The method of claim 70 including determining that the true current zerocondition occurs at the output of one of the cycloconverters when all ofthe naturally commutated switching devices of that cycloconverter arenon-conducting.
 72. The method of claim 71 including sensing thevoltages across the naturally commutated switching devices of eachcycloconverter and determining that all the naturally commutatedswitching devices of one of the cycloconverters are non-conducting whenthe voltages across all of the naturally commutated switching devices ofthat cycloconverter are above a predetermined level.
 73. The method ofclaim 72 including bandpass filtering the instantaneous output currentof each of the cycloconverters to produce a filtered signal from thatinstantaneous output current to reduce current ripple and ensure that afundamental frequency component of each filtered signal does not haveany phase-shift relative to the instantaneous output current of thecycloconverter being filtered, and comparing each of filtered signals toat least one reference level to determine whether the instantaneousoutput current of the corresponding cycloconverter transitioned frompositive to negative or from negative to positive.
 74. The method ofclaim 73 wherein the reference level includes first and second referencelevels of +0.1 A and −0.1 A and the fundamental frequency is 60 Hz. 75.The method of claim 66 wherein coupling the outputs of the first andsecond voltage sources in one of parallel and series includes switchinga single pole switch coupled across the outputs of the first and secondvoltage sources open to couple the outputs in series and closed tocouple the outputs in parallel.
 76. The method of claim 75 wherein thesingle pole switch is a single pole relay.
 77. The method of claim 70wherein a switch is coupled between the outputs of the first and secondvoltage sources, the method including: switching the generator systemfrom the first mode to the second mode by first opening the switch, thendisabling the naturally commutated switching devices of the first andsecond cycloconverters so that they are all non-conducting, and after apredetermined delay, reenabling the naturally commutated switchingdevices of the first and second cycloconverters and operating the firstand second cycloconverters so that the voltages produced at the outputsof the first and second cycloconverters are one-hundred and eightydegrees out of phase with each other, and switching the generator systemfrom the second mode to the first mode by disabling the naturallycommutated switching devices of one of the first and secondcycloconverters, reenabling after a predetermined delay the naturallycommutated switching devices that were disabled and operating the firstand second cycloconverters so that the voltages produced at the outputsof the first and second cycloconverters are in-phase, and then closingthe switch.
 78. The method of claim 68 wherein generating the rotorposition signal indicative of the position of the rotor of the permanentmagnet generator includes generating rotor position signals that aredisplaced one-hundred and twenty degrees from each other.
 79. The methodof claim 78 wherein generating the rotor position signals includesgenerating them with hall effect transducers.
 80. In a dual modegenerator system having a nominal output voltage of 120 VAC when it isin a first mode and nominal output voltages of both 120 VAC and 240 VACwhen it is in a second mode, the generator system having a permanentmagnet generator having first and second sets of isolated three-phasewindings and a rotor, an engine for driving the rotor of the permanentmagnet generator, a plurality of rotor position sensors that generatesignals indicative of the position of the rotor that are displacedone-hundred and twenty degrees from each other, a first cycloconvertercoupled to the first set of windings and a second cycloconverter coupledto the second set of windings, each cycloconverter having a live outputand a neutral output, the live output of the first cycloconvertercoupled to a live output of a first outlet and the neutral output of thefirst cycloconverter coupled to a neutral output of the first outlet,the live output of the second cycloconverter coupled to a live output ofa second outlet and the neutral output of the second cycloconvertercoupled to a neutral output of the second outlet, the first and secondcycloconverters each having a positive bank of naturally commutatedswitching devices and a negative bank of naturally commutated switchingdevices, a first filter capacitor coupled across the live output andneutral output of the first cycloconverter and a second filter capacitorcoupled across the live output and neutral output of the secondcycloconverter, a third outlet coupled across the live outputs of thefirst and second outlets and a switch coupled across the live outputs ofthe first and second outlets in parallel with the third outlet, a methodof operating the dual mode generator system, comprising using thesignals indicative of the position of the rotor of the permanent magnetgenerate to develop cosine control waves to control the switching of thenaturally commutated switching devices of the first and secondcycloconverters, operating the generator system in the first mode withthe switch closed coupling the live outputs of the first and secondcycloconverters in parallel and operating the first and secondcycloconverters so that output voltages at their live outputs are inphase where the nominal 120 VAC output voltage is produced at the firstand second outlets with a greater available current than available whenthe generator system is in the second mode; and operating the generatorsystem in the second mode with the switch open coupling the live outputsof the first and second cycloconverters in series and operating thefirst and second cycloconverters so that the output voltages at theirlive outputs are one-hundred and eighty degrees out of phase where thenominal 120 VAC output voltage is produced at the first and secondoutlets and the 240 VAC output voltage is produced at the third outlet.81. The method of claim 80 wherein the generator system includes anengine for driving the rotor of the permanent magnet generator, themethod including using the rotor position signals to drive the permanentmagnet generator as a brushless DC motor to start the engine.
 82. Themethod of claim 80 including generating a reference wave, comparing thecosine control waves to the reference wave, and controlling switching ofthe naturally commutated switching devices of the first and secondcycloconverters based on the comparisons of the cosine control waves tothe reference wave.
 83. The method of claim 81 including operating thenaturally commutated switching devices of the positive and negativebanks of each cycloconverter in a non-circulating mode and, for eachcycloconverter, enabling one of the positive and negative banks of thatcycloconverter and disabling the other of the positive and negativebanks of that cycloconverters based on the instantaneous output currentof that cycloconverter transitioning between positive and negative orbetween negative and positive and enabling that one of the positive andnegative banks that is being enabled only after a true current zerocondition occurs at the live output of that cycloconverter.
 84. Themethod of claim 83 including determining for each cycloconverter thatthe true current zero condition occurred at the live output of thatcycloconverter when all the naturally commutated switching devices ofthat cycloconverter are non-conducting.
 85. The method of claim 84including for each cycloconverter sensing the voltages across thenaturally commutated switching devices of that cycloconverter anddetermining that all the naturally commutated switching devices of thatcycloconverter are non-conducting when all the voltages across all thenaturally commutated switching devices of that cycloconverter are abovea predetermined level.
 86. The method of claim 83 including for eachcycloconverter bandpass filtering the instantaneous output current ofthat cycloconverter to produce a filtered signal to reduce currentripple and ensure that a 60 Hz fundamental frequency component of thefiltered signal does not have any phase-shift relative to theinstantaneous output current of that cycloconverter, and comparing thefiltered signal to at least one reference level to determine whether theinstantaneous output current of that cycloconverter transitioned frompositive to negative or from negative to positive.
 87. The method ofclaim 86 wherein the reference level includes first and second referencelevels of +0.1 A and −0.1 A.
 88. The method of claim 80 wherein theswitch is a single pole relay.
 89. The method of claim 80 including:switching the generator system from the first mode to the second mode byfirst opening the switch, then disabling the naturally commutatedswitching devices of the first and second cycloconverters so that theyare all non-conducting, and after a predetermined delay, reenabling thenaturally commutated switching devices of the first and secondcycloconverters and operating the first and second cycloconverters sothat the voltages produced at the live outputs of the first and secondcycloconverters are one-hundred and eighty degrees out of phase witheach other, and switching the generator system from the second mode tothe first mode by disabling the naturally commutated switching devicesof one of the first and second cycloconverters, reenabling after apredetermined delay the naturally commutated switching devices that weredisabled and operating the first and second cycloconverters so that thevoltages produced at their live outputs are in-phase, and then closingthe switch.
 90. A method of controlling a generator system having an ACgenerator with an output coupled to a cycloconverter, the cycloconverterhaving a positive bank of naturally commutated switching devices and anegative bank of naturally commutated switching devices, the methodcomprising developing control waves based upon the position of the rotorand using the control waves to control switching of the naturallycommutated switching devices.
 91. The method of claim 90 wherein thegenerator system includes a plurality of rotor position sensors thatgenerate signals indicative of the position of the rotor that aredisplaced one-hundred and twenty degrees from each other, the methodincluding using the rotor position signals to generate the controlwaves.
 92. The method of claim 91 including generating a reference wave,comparing the control waves to the reference wave and generating firingsignals for the naturally commutated switching devices based oncomparisons of the control waves to the reference wave.
 93. The methodof claim 92 wherein the generator system includes an engine for drivingthe AC generator, the method including driving the AC generator as abrushless DC motor to start the engine and using the rotor positionsignals in doing so.
 94. The method of claim 91 including operating thepositive and negative banks of the cycloconverter in a non-circulatingmode and enabling one of the positive and negative banks and disablingthe other of the positive and negative banks based on the instantaneousoutput current of the cycloconverter produced at an output of thecycloconverter transitioning between positive and negative or betweennegative and positive wherein the one of the positive and negative banksbeing enabled is enabled only after a true zero current condition occursat the output of the cycloconverter.
 95. The method of claim 94including determining that the true current condition occurs when all ofthe naturally commutated switching devices are non-conducting.
 96. Themethod of claim 95 including sensing the voltages across the naturallycommutated switching devices and determining that all the naturallycommutated switching devices are non-conducting when the voltages acrossall of the naturally commutated switching devices are above apredetermined level.
 97. The method of claim 96 including bandpassfiltering the instantaneous output current of the cycloconverter toproduce a filtered signal to reduce current ripple and ensure that afundamental frequency component of the filtered signal does not have anyphase-shift relative to the instantaneous output current, and comparingthe filtered signal to at least one reference level to determine whetherthe instantaneous output current transitioned from positive to negativeor from negative to positive.
 98. The method of claim 97 wherein thereference level includes first and second reference levels of +0.1 A and−0.1 A.
 99. A method of controlling a generator system having an ACgenerator with an output coupled to a cycloconverter, the cycloconverterhaving a positive bank of naturally commutated switching devices and anegative bank of naturally commutated switching devices, the methodcomprising operating the positive and negative banks of thecycloconverter in a non-circulating mode and enabling one of thepositive and negative banks and disabling the other of the positive andnegative banks based on the instantaneous output current of thecycloconverter produced at an output of the cycloconverter transitioningbetween positive and negative or between negative and positive, andbandpass filtering the instantaneous output current of thecycloconverter to produce a filtered signal to reduce current ripple andensure that a fundamental frequency component of the filtered signaldoes not have any phase-shift relative to the instantaneous outputcurrent, and comparing the filtered signal to at least one referencelevel to determine whether the instantaneous output current transitionedfrom positive to negative or from negative to positive.
 100. The methodof claim 99 including enabling the one of the positive and negativebanks being enabled only after the other of the positive and negativebanks has been disabled and a true zero current condition occurs at theoutput of the cycloconverter.
 101. The method of claim 100 includingdetermining that the true current condition occurs when all of thenaturally commutated switching devices are non-conducting.
 102. Themethod of claim 101 including sensing the voltages across the naturallycommutated switching devices and determining that all the naturallycommutated switching devices are non-conducting when the voltages acrossall of the naturally commutated switching devices are above apredetermined level.
 103. The method of claim 99 wherein the fundamentalfrequency is 60 Hz.
 104. The method of claim 99 wherein the referencelevel includes first and second reference levels of +0.1 A and −0.1 A.105. The generator system of claim 35 wherein the controller includes afirst controller for controlling the first cycloconverter and a secondcontroller for controlling the second cycloconverter.
 106. The generatorsystem of claim 55 wherein the controller includes a first controllerfor controlling the first cycloconverter and a second controller forcontrolling the second cycloconverter.
 107. The generator system ofclaim 10 wherein the controller simulates back emf voltage waveforms ofthe generator using the rotor position signal and develops control wavesfrom the back emf voltage waveforms which it uses to control thecycloconverters.
 108. The generator system of claim 12 wherein thecontroller simulates back emf voltage waveforms of the generator usingthe rotor position signals and develops control waves from the back emfvoltage waveforms which it uses to control the cycloconverters.
 109. Thegenerator system of claim 35 wherein the controller simulates back emfvoltage waveforms of the generator using the rotor position signal anddevelops the control waves from the back emf voltage waveforms.
 110. Thegenerator system of claim 36 wherein the controller simulates back emfvoltage waveforms of the generator using the rotor position signals anddevelops the control waves from the back emf voltage waveforms.
 111. Thegenerator system of claim 55 wherein the controller simulates back emfvoltage waveforms of the generator using the rotor position signals anddevelops the cosine control waves from the back emf voltage waveforms.112. The method of claim 67 including simulating back emf voltagewaveforms of the generator using the rotor position signals anddeveloping the control waves from the back emf waveforms.
 113. Themethod of claim 80 including simulating back emf voltage waveforms ofthe generator using the rotor position signals and developing the cosinecontrol waves from the back emf waveforms.
 114. The method of claim 90including simulating back emf voltage waveforms of the generator basedupon the position of the rotor and developing the control waves based onthe back emf voltage waveforms.
 115. The generator system of claim 19and further including a portable universal battery pack coupled to thebrushless DC motor drive circuit that provides DC power to the brushlessDC motor drive circuit.
 116. The generator system of claim 20 andfurther including a portable universal battery pack coupled to thebrushless DC motor drive circuit that provides DC power to the brushlessDC motor drive circuit.
 117. The generator system of claim 39 andfurther including a portable universal battery pack coupled to thebrushless DC motor drive circuit that provides DC power to the brushlessDC motor drive circuit.
 118. The generator system of claim 56 andfurther including a portable universal battery pack coupled to thebrushless DC motor drive circuit that provides DC power to the brushlessDC motor drive circuit.
 119. The method of claim 69 further includingusing a portable universal battery pack coupled to the brushless DCmotor drive circuit to supply DC power to the brushless DC motor drivecircuit.
 120. The method of claim 81 and further including using DCpower of a portable universal battery of the generator system instarting the engine.
 121. The method of claim 93 and further includingusing DC power of a portable universal battery of the generator systemin starting the engine.